JP2011-1692232A discloses a spark-ignited internal combustion engine which has a ceramic-based heat shielding layer on a bottom surface of a cylinder head and a top surface of a piston. In this engine, both the bottom surface of the cylinder head and the top surface of the piston include a squish surface and a non-squish surface. The squish surface is a surface which forms a squish area. The squish area is a narrow space which is formed during the compression cycle by the bottom surface of the cylinder head and the top surface of the piston. The non-squish surface is a surface which does not form the squish area.
In the engine, the heat shielding layer is formed on the non-squish surface. Here, the non-squish surface is closer to the spark plug than the squish surface. When the heat shielding layer is formed on the non-squish surface, it is possible to suppress a heat release from a surface close to the spark plug. In this engine, however, the heat shielding layer is not formed on the squish surface but base material of the cylinder head and the piston is exposed. When the base material of the squish surface is exposed, it is possible to enhance the heat release from a surface far from the spark plug.
JP2017-066996A discloses a spark-ignited internal combustion engine which has two different heat shielding layers on a top surface of a piston. In this engine, the top surface of the piston includes the squish surface and the non-squish surface. On the squish surface, a heat shielding layer which includes ceramics having a hollow structure and a silicon-based binder is formed. On the other hand, a ceramic-based heat shielding layer obtained by spraying is formed on the non-squish surface. According to such the two heat shielding layers, it is possible to suppress the heat release from the squish and non-squish surfaces. In addition, it is possible to increase a heat shielding performance of the non-squish surface than that of the squish surface.
The heat shielding layers mentioned above is able to apply to a compression self-ignited internal combustion engine. The present inventor has been investigated porous alumina (hereinafter also referred to as “alumite”) as the heat shielding layer applied to the compression self-ignited internal combustion engine. The alumite is obtained by anodization of the base material of the cylinder head and the piston. The alumite has a lower thermal capacity per unit volume than a general ceramic-based heat shielding layer. Therefore, when the alumite is formed on the top surface of the piston or the bottom surface of the cylinder head, it is possible to enhance a tracking performance of a surface temperature of the heat shielding layer with respect to a gas temperature in a cylinder.
In addition, the alumite has a lower thermal conductivity than the general ceramic-based heat shielding layer. Actually, it was found by the present inventor that a fuel consumption rate in a low-load region of the engine is greatly improved when the alumite is formed on the top surface of the piston or the bottom surface of the cylinder head. However, at the same time, it was found that the fuel consumption rate in a high-load region of the engine is not improved as expected.
The present disclosure addresses the above described problem, and an object of the present disclosure is, to provide a technology to improve the fuel consumption rate in the high-load region of the compression self-ignited internal combustion engine when porous alumina is applied to the engine as the heat shielding layer.